I wrote this mainly because someone asked how to do this in PHP. Turns out there is nothing in like the venerable URI module in all of PHP-dom (how pathetic is that for a language as web-centric as it?).

First, I tried writing this with mostly string functions, but that was too painful, so I switched to doing it using regexes, even though they’re piss-poorly supported. After a while, my eyes glazed over, so I resorted to writing and debugging the code in Perl first, and then compiling it down to machine code PHP.

I thought I’d post this byproduct here for posterity, in case someone needs it in some circumstance.

Tests included, but I’m pretty sure they’re not extensive enough to be 100% confident in the code. Proceed at your discretion.

sub abs_url {
    my ( $relative, $base ) = @_;

    return $relative if $relative =~ m{ \A http:// }ix;

    my ( $host, $hostrelative_abs ) = $base =~ m{
        http:// # skip scheme
        ([^/]*) # capture hostname
        /*      # skip front slashes
        (.*?)   # capture everything that follows, but
        [^/]*   # leave out the optional final non-directory component

    $hostrelative_abs = '' if $relative =~ m!^/!;

    my $abs_url = join '/', $host, $hostrelative_abs, $relative;

    # replace '//' or '/./' with '/'
    1 while $abs_url =~ s{ / \.? (?=/|\z) }{}x;

    # remove '/foo/..' (but be careful to skip '/../..')
    1 while $abs_url =~ s{ / (?!\.\.) [^/]+ / \.\. (?=/|\z) }{}x;

    return "http://$abs_url";
use Test::More;
my @test = qw(
 /foo/bar.gif    http://www.exam
 foo/bar.gif    http://www.exam
 ../../bar.gif    http://www.exam
 foo/bar.gif     http://www.exam
 ../foo/bar.gif     http://www.exam
 ../../foo/bar.gif http://www.exam
plan tests => @test / 3;
do { is abs_url( $test[0], $test[1] ), $test[2]; splice @test, 0, 3 } 
+while @test;